Caption: Knee replacement. Coloured X-ray of a prosthetic knee implant (yellow/purple), of a female patient's left leg (rear view). The implant is attached by pegs (at upper and lower centre) into the femur and tibia bones (brown) respectively, with surgical cement. The flexible joint (centre) can hinge like the old joint. The patient's kneecap (patella, not seen) was not replaced. The implant replaced the old joint that had become damaged or lost cartilage, possibly due to an accident or osteoarthritis. Healthy cartilage reduces friction between the bones, and its progressive loss causes joint pain and immobility.